Defined dedicated namespace for the zrtp-hash element to improve service discovery and handling of session-info message; clarified protocol flow and security considerations.
0.0.12009-02-17psa
First draft, copied from XEP-0167.
Jingle RTP Sessions (XEP-0167)XEP-0167: Jingle RTP Sessions <https://xmpp.org/extensions/xep-0167.html>. recommends the use of the Secure Real-time Transport Protocol (SRTP) for end-to-end encryption of RTP sessions negotiated using Jingle (XEP-0166)XEP-0166: Jingle <https://xmpp.org/extensions/xep-0166.html>.. An alternative approach to end-to-end encryption of RTP traffic is provided by RFC 6189RFC 6189: ZRTP: Media Path Key Agreement for Unicast Secure RTP <http://tools.ietf.org/html/rfc6189>.. Although negotiation of ZRTP mainly occurs in the media channel rather than the signalling channel, the ZRTP specification defines one SDP attribute called "zrtp-hash" (this communicates the ZRTP version supported as well as a hash of the Hello message).
If either party to a Jingle RTP session wishes to use ZRTP, the party SHOULD send a Jingle session-info message that includes a <zrtp-hash/> element qualified by the 'urn:xmpp:jingle:apps:rtp:zrtp:0' namespace, as shown in the following example.
In accordance with XEP-0166, if the receiving party does not understand the payload of the session-info message then it MUST return a <feature-not-implemented/> error.
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However, if the receiving party also supports and wishes to use ZRTP, it too SHOULD send a session-info message containing a zrtp-hash element.
If an entity supports the zrtp-hash session-info message, it MUST advertise that fact in its responses to Service Discovery (XEP-0030)XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>. information ("disco#info") requests by returning a feature of "urn:xmpp:jingle:apps:rtp:zrtp:0":
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In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115)XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
Security considerations for ZRTP itself are provided in draft-zimmermann-avt-zrtp.
XMPP stanzas such as Jingle session-info messages and service discovery exchanges are not encrypted or signed. As a result, it is possible for an attacker to intercept these stanzas and modify them, thus convincing one party that the other party does not support ZRTP and therefore denying the parties an opportunity to use ZRTP. However, because the zrtp-hash is mostly advisory, the parties could still use ZRTP even if the signalling channel is compromised.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA)The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>..
See the XML Schemas section of this document.
If this specification is advanced to a status of Draft, the XMPP Registrar shall add the following element definition to the schema for the 'urn:xmpp:jingle:apps:rtp:info:1' namespace defined in XEP-0167: